Transfer of zinc vapour from a furnace to a condenser



NSERy s. E. wooDs ETAL Filed May 2o. l1957 TRANSFER OF ZINC VAPOUR FROM A FURNACE T0 A CONDE Nov. 10, 1959 2,912,322 Patented Nov. 1o, 1959 TRANSFER F ZINC VAPOUR FROM Av TO A CONDENSER Stephen Esslemont Woods and vLeslie JackDerham, Bristol, Gloucester, England, assignors to Metallurgical Processes Limited, Nassau, Bahamas, and The National Smelting Company Limited, London, England Application May 20, 1957, Serial No. 660,237 Claims priority, application Great: Britain .Tune 13, 1956 5 Claims. (Cl. 75-88) This invention relates to the transfer of zinc vapour from a furnace to a condenser, more especially when the furnace oiftake gases contain carbon dioxide. It is particularly applicable in zinc blast furnaces, in which the carbon dioxide content is generally in the region of 5-12%, with 5-7% zinc. It is useful also in electrothermic furnaces, Where the carbon vdioxide content is lower, but the zinc content, i's higher. L

Zinc vapour and carbon dioxide can co-existinstable equilibrium only in presence of carbon monoxide and above a certain critical temperature, which'depend's on the gas composition according tothe equilibrium constant of the reaction.

ZnO (solid) -I-CO(gas) =Zn(gas) -l-CO2(gas) (l) This equilibrium constant can be represented as: i

(Zn) (C02) K (C0) where (Zn), (CO2) and (CO) represent the partial Ipressure in atmospheres of these gases.` K .varies'With temperature as follows: I

the Iower the temperature at which the gases are'withdrawn from the smelting charge, the lower is the amount:A of lead sulphide vapor carried by the gases, and hence that it was desirable that the gases should leave the furnace charge at a temperature only slightly above the equilibrium temperature. But while this alleviated the sulphur trouble, it accentuated the tendency for zinc vapor to oxidize during the transfer of the gases to the condenser. Accordingly, the patentees propose (1) to reduce the amount of lead sulphide vapor in the furnace gases by so controlling the conditions of smelting that the gases leave the furnace change at a temperature slightly above the equilibrium temperature, and (2) to insurel that the gases remain above this equilibrium temperature during the whole course of their passage from the furnace to the condenser by introducing oxygen into the gases for combustion with carbon monoxide therein and so raise the temperature of the gases above their initial temperature and well above the equilibrium temperature at which zinc vapor couldreact with carbon dioxide.

The high temperature at the top of a furnace charge, re-

quired by the consideration above, is objectionable in a K Temperature, C.

With a gas containing 5.3% Zn, 10% CO2. and 20% CO, K=0.053X0.l0/0.20=0.0265, so that the equilibrium temperature vis 995 C. Such a gas might be produced from a blast furnace. Froman electrothermic furnace a gas containing 40% Zn, 2% CO2`and 50% CO,

gives K=0.4 0.02/0.5=0.0l6, soY that theequilibrium temperature is 959 C. i j

The equilibrium temperature belowwhich zinc; vapour can be oxidised by carbon dioxide is usually between 900 C. and 1000 C. varying with the composition of the gases.

As explained in the United States patent of Morgan et al. No. 2,816,022 (British Patent No. 741,258), when a blast furnace is operated for smelting a zinciferous material so as to produce molten lead bullion in addition to zinc vapor, it is important to avoid the volatilization of a large amount of lead sulphide, since volatilzed lead sulphide not only contains lead that could otherwise have been tapped from the bottom of the furnace but also acts to foul the condenser, thus necessitating its shut-down for cleaning operations. The patentees had found that blast furnace run with a viewto obtaining lead bullion in addition to zinc, since there is an undue volatilization oflead sulphide. Even if the gases leave the charge at only slightly above the equilibrium temperature (which, as already explained, may be as high as 1000 C.), there is still, in a furnace producing both lead and zinc, sufficient volatilization of lead sulphideto cause appreciable fouling of the condenser.

We have discovered that the reaction:

that is, the reverse of Reaction 1, takes place only on the walls of the offtake flue, and that when-.these walls are kept at a temperature at which the Zinc vapor and carbon dioxide contained in the gases can remain in stable equilibrium in the presence of the carbon monoxide contained therein no appreciable amount of zinc oxide is formed by reaction of zinc vapor with carbon dioxide, even though the gasespassing through the ue are at a temperature below the equilibrium temperature.

We have made thefurther discovery that when vthe walls of the oiftake flue are heated to above the equilibrium temperature, the furnace may be operated so that the gases leave the charge at a temperature well below the equilibrium temperature, e.g. within the range of 800 to 850 C. v e Our invention accordingly resides in a method of -pro` ducingzinc by smelting a zinciferous material in a fur'- nace and producing av gaseous product containing zinc vapor, carbon monoxide and carbon dioxide, and recovcondenser through a flue, the walls of which'are heated to a temperature above the equilibrium temperature, while the temperature of the gases passing therethrough remains below this equilibrium temperature.

In practicing the invention, use is made of a flue arrangement for transferring the gases containing zinc vapor from the zinc smelting furnace to a condenser in which the walls of the flue are adapted to be heated so as to be maintained at a temperature above the equilibrium temperature at which the zinc vapor and the carbon dioxide in the gases passing through the flue remain in stable equilibrium in the presence of the carbon monoxide therein. When such a heated ue is provided for a blast furnace smelting only zinc (that is not smelting lead along with the zinc), the temperature at the top of the furnace charge may be reduced to a much lower value than has heretofore been thought necessary in order to avoid reoxidation of zinc vapor in -its transfer from the furnace to the condenser.

In order to ensure that the gases leave the furnace at the desired temperature (generally 800 C.-850 C.) the gases are withdrawn from the top level of the charge, and the metalliferous portion of the charge is not too highly preheated; it may be introduced cold or it may be preheated to a temperature not exceeding 600 C. Coke in the charge is preferably highly preheated to 800 C.- 1000 C.

An advantage of reducing the temperature at which the gases leave the charge to a value well below the equilibrium temperature is that the volatilisation of lead sulphide is further reduced. Other advantages are that the amount of lead metal volatilized is reduced, as is also the amount of arsenic volatilized.

The walls of the oft'take ue may be heated to, say, 1000 C. or over, well above the temperature of the gases passing therethrough.

Preferably the flue is made of heat and corrosion-resisting metal.

The invention will be further described with reference to the accompanying drawings.

Figure 1 is a vertical cross-section on the centre line of a suitable furnace, flue and condenser for practicing the invention.

Figure 2 is a section on the line A-A of Figure l.

The furnace 1 is connected with the condenser 2 by means of the flue 3 which is of an alloy steel with 18.2% chromium, 38.1% nickel, 2.48% silicon, 1.02% manganese, 0.08% cobalt and 0.08% carbon. Hot gases produced by the burners 4 are passed around the flue 3 in the combustion chamber 5 to heat it and escape up a waste gas offtake stack 6 under the control of a damper 7.

The flue itself may be freed from blockages, should they occur, by removal of the access plate 8.

The Hue 3 is of a T shape with a generally horizontal section leading to a generally vertical section and is made of circular cross-section tubing. At its intake end the tubing of the ue 3 is welded to a bell-mouthed part 3a of a similar alloy which seals chamber 5 from the: furnace 1.

At the top end of the inclined cross-member of the T the tubing o-f the iiue is welded to a supporting plate 3b which rests on the brickwork while at its lower end the tubing is welded to a flanged plate 3c, the flange of which dips into a circular trough 9 to form a slot between chamber 5 and condenser 2 and allow for expansion.

The furnace gases are still well below their re-oxidation temperature when they have passed through the heated flue.

Thus in one experiment the gases were heated by only 10-15 C. during their passage through the flue.

It will be understood that other means of heating the flue may be used. Thus it may be heated by electrical-resistance heating or by electric-induction heating.

We claim:

1. In the method of producing zinc by smelting a zinciferous material in a furnace with the production of a gaseous product containing zinc vapor, carbon monoxide and carbon dioxide which is transferred through a ue to a condenser for condensation of metallic zinc, the improvement which comprises withdrawing the gaseous product from the smelting charge at a temperature substantially below the equilibrium temperature at which the zinc vapor and the 'carbon 'dioxide remain in equilibrium in the presence of the carbon monoxide, at which temperature there is substantial vreact-ion between the zinc and the carbon dioxide of the gas in contact with a solid surface, and heating the walls of the flue through which the gaseous product is transferred from the furnace to the condenser to a temperature above said equilibrium temperature While maintaining the temperature of the gaseous product delivered to the condenser substantially below said equilibrium temperature, whereby reaction of the carbon dioxide with the zinc is suppressed while said gases are passing through said ue and in contact therewith.

2. A method as claimed in claim 1 in which the walls of the flue are heated to a temperature of at least 1000" C.

3. A method as claimed Vin claim 1 in which the metalliferous portion of the furnace charge is preheated to a temperature not exceeding 600 C. and the coke portion of the charge is preheated to 800 C. to 1000 C.

4. ln the method of producing zinc by smelting a leadcontaining zinciferous material in a blast furnace with the production of a gaseous product containing zinc vapor, carbon lmonoxide and carbon dioxide which is transferred through a flue to a condenser for condensing the zinc vapor to metallic Zinc, the improvement which comprises minimizing the amount of lead sulphide vapor in said gaseous product by controlling the conditions of smelting to withdraw the product from the smelting charge at a temperature within the range of 800 to 850 C` and substantially below the equilibrium temperature at which the zinc vapor and carbon dioxide remain in equilibrium in the presence of carbon monoxide, at which temperature there is substantial reaction between the zinc and the carbon dioxide of the gases in contact with a solid surface, and heating the walls of the ilue through which the gaseous product is transferred from the furnace to the condenser to a temperature above said equilibrium temperature while maintaining the temperature of the gaseous product delivered to the condenser substantially below said equilibrium temperature, whereby reaction of the carbon dioxide with the Zinc is suppressed, while certain gases are passing through said ue and in contact therewith.

5. The improvement according to claim 4, in which the temperature of the gaseous product delivered to the condenser does not exceed about 15 C. above the temperature at which the gaseous product is withdrawn from the smelting charge.

References Cited in the le of this patent UNITED STATES PATENTS 2,662,764 Arut'undoff Dec. l5, 1953 2,668,760 Breyer et al. Feb. 8, 1954 2,676,010 Matthies Apr. 20, 1954 2,682,462 Woods June 29, 1954 2,766,114 Najarian Oct. 9, 1956 2,816,022 Morgan et al Dec. 10, 1957 

1. IN THE METHOD OF PRODUCING ZINC BY SMELTING A ZINCIFEROUS MATERIAL IN A FURNACE WITH THE PRODUCTION OF A GASEOUS PRODUCT CONTAINING ZINC VAPOR, CARBON MONOXIDE AND CARBON DIOXIDE WHICH IS TRANSFERRED THROUGH A FLUE TO A CONDENSER FOR CONDENSATION OF METALLIC ZINC, THE IMPROVEMENT WHICH COMPRISES WITHDRAWING THE GASEOUS PRODUCT FROM THE SMELTING CHARGE AT A TEMPERATURE SUBSTANIALLY BELOW THE EQUILBRIUM TEMPERATURE AT WHICH THE ZINC VAPOR AND THE CARBON DIOXIDE REMAIN IN EQUILIBRIUM IN THE PRESENCE OF THE CARBON MONOXIDE, AT WHICH TEMPERATURE THERE IS SUBSTANTIAL REACTION BETWEEN THE ZINC AND THE CARBON DIOXIDE OF THE GAS IN CONTACT WITH A SOLID SURFACE, AND HEATING THE WALLS OF THE FLUE THROUGH WHICH THE GASEOUS PRODUCT IS TRANSFERRED FROM THE FURNACE TO THE CONDENSER TO A TEMPERATURE ABOVE SAID EQUILIBRIUM TEMPERATURE WHILE MAINTAINING THE TEMPERATURE OF THE GASEOUS PRODUCT DELIVERED TO THE CONDENSER SUBSTANTIALLY BELOW SAID EQUILIBRIUM TEMPERATURE, WHEREBY REACTION OF THE CARBON DIOXIDE WITH THE ZINC IS SUPPRESSED WHILE SAID GASES ARE PASSING THROUGH SAID FLUE AND IN CONTACT THEREWITH. 